1. Field of the Invention
The invention relates to a novel corrosion inhibitor and its use for protecting metallic materials in strongly alkaline medium.
2. Brief Description of the Prior Art
It is known that metallic materials, depending on temperature, pressure and composition, are corroded both by acidic media and by alkaline media. To ensure reliable operation in chemical plants or elsewhere, corrosion rates are therefore determined in laboratory experiments and later checked by measurements on the plant when in operation. Values based on experience set the limits for estimating whether construction of a plant from a certain metallic material is appropriate or what the maintenance interval should be.
In the case of extreme conditions, e.g. elevated temperatures above 50° C. and/or presence of concentrated acidic or alkaline media having a pH of >13, the search for and selection of a suitable metallic material is not easy. Although materials coated on the inside (enamel) are often used in the case of acidic media, this is not possible in the case of alkaline media since enamel is corroded by alkali. During the lifetime of a plant, this leads to costly maintenance or to shutdown of the plant.
Illustrative of the strongly alkaline medium of concern here is sodium hydroxide e prepared by various processes which give differing concentrations in aqueous solutions or melts. Examples are the diaphragm, membrane and amalgam processes. A comparable situation applies to the preparation of potassium hydroxide or other strongly alkaline solutions of alkali metal hydroxides or alkaline earth metal hydroxides. If the concentration of the aqueous sodium hydroxide obtained in the process is too low, it is increased by evaporation. The concentration of NaOH in the membrane process is about 30%. However, the normal commercial concentration of NaOH is about 50%.
Evaporation of NaOH solutions to bring the concentration from 30% to a higher concentration is, therefore, necessary. There are few metallic materials which can safely withstand this evaporation with acceptable damage in this (high-temperature) process. Materials which may be mentioned include, for example, stainless steels or other alloy steels. However, if the sodium hydroxide solution contains, as a result of the process, amounts of chlorate or other oxidants such as oxygen or Fe3+ in the ppm range, the corrosion is considerably increased and operation of an evaporation plant becomes more difficult and more expensive. This applies to plants which are to be operated using membrane alkali or diaphragm alkali. The former contains up to 50 ppm of chlorate, the latter up to 5000 ppm of chlorate. An evaporation plant can then no longer be operated economically because of the increased corrosion or can give concentrations of only up to about 50%, since the process is temperature-dependent. Furthermore, the corrosion leads to the contamination of the sodium hydroxide with metal ions, mostly nickel, manganese, chromium or the like, in each case depending on the metallic material used for the plant. These impurities can then adversely affect or prevent subsequent chemicals or other processes. In virtually all cases, these ions are discharged into the environment sooner or later, which is not desirable.
It is an object of the invention to provide a method which considerably reduces the corrosion of metallic materials by strongly alkaline media, in particular media containing sodium hydroxide.